Thermal barrier coating (TBC) systems have been readily employed on first and second rows of turbine blades and vanes as well as on combustion chamber components exposed to the hot gas path of gas turbines. Typically, yttria stabilised zirconia barrier coatings are extensively applied to the hot sections and provide protection against thermal-mechanical shock, high-temperature oxidation and hot corrosion degradation.
While the primary drive to implement TBCs was initially the lifetime extension of the coated components, advanced gas turbines utilise TBCs more and more to allow increased efficiency and power output of a gas turbine. One measure to improve efficiency and power output is to reduce the cooling air consumption of the components in the hot gas path, i.e. by allowing those components to be operated at higher temperatures. The push to higher firing temperatures and reduced cooling flows generates an ongoing demand for advanced TBCs with higher temperatures, stability and better thermal isolation to achieve long term efficiency and performance goals of advanced gas turbines.
Present day TBCs often comprise a two-layer system: an outer isolating ceramic layer and an underlying oxidation resistant metallic layer (bond coat) deposited directly onto the surface of the metallic component. The bond coat provides the physical and chemical bond between the ceramic coating and the substrate and serves as an oxidation and corrosion resistant by forming a slow growing adherent protective alumina scale. The top ceramic layer provides benefits in performance, efficiency and durability through a) increased engine operating temperature b) extended metallic component lifetime when subjected to elevated temperature and stress and c) reduced cooling requirements for the metallic components. Depending on the ceramic layer thickness and through thickness heat flux, substrate temperatures can be reduced by several hundred degrees.
The development and acceptance of TBCs are closely linked to processing technology: in this connection, ceramic topcoats are presently deposited using air plasma spraying (APS) or electron beam physical vapour deposition (EB-PVD) processes. Although both coatings have the same chemical composition, their microstructures are fundamentally different from each other as are their thermal isolation properties and performances.
The desired increase in operating temperature is accomplished to a great extent by taking credit of the superior temperature capability of the ceramic TBC system in conjunction with its excellent thermal isolation behavior due to its low thermal conductivity. Improvement of the thermal isolation of the TBC can be achieved by increasing the TBC thickness, modification of the TBC microstructure (e.g. porosity) or by using materials with lower bulk thermal conductivity.